Telecommunication system



1946- P. E. A. cOwLEY ET AL 2,411,415

TELEGOMMUNICATION SYSTEM Filed Dec. 3, 1945 Low P ss FILTER I Low P ssFILTER I 11 I ALP 1 ALP 2 DIREcTIONALcARRIER 5 P1 5 P 2 DIREcTIONALCARRIER FILTER & AMPLIFIER FILTER & cONTROL AMPLIFIER 10 1 12 BAND-PASSRECEIVING BAND-PAss RECEIVING FILTERs FILTERs OscILLATOR- Po PRF PILOTRECEIVING FILTER SENDING END PA PILOT FREQUENCY AMPLIFIER w PILOTFREQUENCY F1 1 PLI LEvEL INDICATING EQUIPMENT RECEIVING END F: COWZeyarzzZ B E: 6. W3 25 Patented Nov. 19, 1946 TELECOMMUNICATION SYSTEMPercy Edward Alexander Cowley, Coventry, and

Eric Cliiford Walls, Liverpool, England, assignors to AutomaticTelephone & Electric Company Limited, Liverpool, England, a Britishcompany Application December 3, 1943, Serial No. 512,816 In GreatBritain January 20, 1943 4 Claims. 1

The present invention relates to telecommunication systems particularlythose employing carrier current, and finds advantageous application tosystems in which, for gain control purposes, a pilot frequency istransmitted over the same medium as the carrier speech and/or signallingfrequencies and has a frequency value of the same order thereof. Thechief object of the invention is to minimise the adverse effect ofambient temperature variation on the characteristics of the filtersemployed which in the particular application referred to may upset thecontrol function of the pilot frequency.

In such systems the pilot frequency in following the same course or pathas the carrier frequencies is subjected to the same transmission loss.Thus any common apparatus such as line or directional filters in saidpath may have a transmission loss which varies with ambient temperatureWithout affecting the relative received levels of the pilot and carrierfrequencies. At the receiving end, where filters are employed toseparate the pilot from the carrier frequencies, it is essential forsatisfactory operation that the filters individual to said carrier andpilot frequencies shall have equal loss/temperature coefficients,- thatis to say, that the respective losses with increase in temperature shalleither remain constant or shall increaseto an equal extent.

It is found that the band pass carrier receiving filters havesubstantially zero loss/temperature co-efficients, but the pilotreceiving filter has, due to its highly selective nature a considerablepositive loss/temperature co-efficient. The invention provides a simplemethod of compensation whereby a substantially zero loss/temperatureco-efficient can be obtained for the pilot frequency receivingequipment.

According to one feature of the invention, in a telecommunication systememploying filter circuits the attenuating effect of which varies withtemperature, a compensating arrangement is provided in which anamplifier employed in conjunction with a filter circuit is arranged tohave a positive gain/temperature coefficient of such value as tocompensate substantially completely for the positive loss/temperaturecoefficient of the filter.

According to another feature of the invention, in a telecommunicationsystem employing a pilot frequency transmitted over the maincommunicoeificient of the filter for the pilot frequency compared withthat of the filter for the main channel is counteracted by including inthe pilot frequency receiving circuit an amplifier arranged to have apositive gain/temperature coefiicient.

The invention will be better understood from the following descriptionof one method of carrying it into effect which should be taken inconjunction with the accompanying drawing comprising Figs. 1 and 2. Fig.1 shows in block schematic form speech side-band and pilot frequencytransmitting and receiving arrangements in a typical multi-channelcarrier telecommunication system using single side-band speechtransmission and employing different channels for transmission in eachdirection over an ordinary telephone line also used foraudio frequencytransmission. Fig. 2 shows circuits of a onevalvetemperature-compensated pilot frequency amplifier of the negativefeedback type, the feedback circuit incorporating resistance elementshaving different temperature co-efiicients.

Referring 'now to Fig. 1, It represents an audio-frequency telephoneline on which a multichannel carrier system is superimposed. At thesending and receiving ends SE and RE, there are inserted the audiofrequency low-pass filters ALPI and ALPZ which serve to exclude thecarrier and pilot frequenciesfrom the sending and receiving audiofrequency telephone apparatus. At the sending end SE, carrier frequencyspeech is fed to one of a number of leads such as 10 from the modulatingequipments, not shown, while the pilot frequency is generated byftheoscillator P0. The equipment common to the .pilot and speech side-bandfrequencies is designated SPI and includes the directionai carrierfilter together with amplifying quipment. The

equipmentindividual to the speech side-bands is designated SI andincludes the bandpass single side-band sending filters.

At the receiving end RE, SP2 designates the equipment common to thepilot frequencies and speech side-bands and includes the directionalcarrier filter together with a control amplifier whichis normally set toprovide a zero overall gain, but the gain of which'is adjustable inaccordance with the received pilot frequency level.

The equipment individual to the speech sidebands is designated S2 andincludes band-pass receiving filters via which the received sidebandsare directed over a lead such as: IE to their appropriate receivingdemodulators. The highly selective pilot receiving filter is designatedPRF, the succeeding pilot frequency amplifier is desig- 3 hated PA,while PLl represents pilot frequency level indicating equipment by meansof which the gain of the common control amplifier in equipment SP2 canbe manually or automatically adjusted in accordance with the receivedpilot frequency level. The positive loss/temperature coefficient of thepilot receivingfilter PRF is considerable owing particularly to thenature of this filter, while that of the band-pass speech sidebandreceiving filters S2 is substantially zero. In order to match theloss/temperature characteristics of the pilot frequency receivingequipment with that of the speech side-band receiving equipment, it isarranged that the pilot frequency amplifier PA is given a positivegain/temperature co-efiicient which will compensate for.

the positive loss/temperature co-efiicient of the preceding pilotreceiving filter PRF.

One suitable form of temperature-compensated amplifier is shown in Fig.2 where leads l3 and i l extend from the pilot receiving filter and theoutput leads I5 and I6 extend to the pilot level indicator. The grid ofvalve V automatically receives a standing negative bias by virtue of thebiassing resistance RI pass condenser Ci, while one of the two secondarywindings of the output transformer T2 is centretapped and is connectedup with two resistance elements R and S so as to form a bridge network,

across the centre points of which a feedback cir-' cuit is taken via thesecondary winding of input transformer TI to the valve grid. Resistanceelement R is made up of one of theknown forms of copper/nickel alloywhich has a zero temperature co-efiicient, while resistance element S.utilises iron wire which has a considerable positive temperatureco-efiicient.

At some temperature which is preferably well above that to which theapparatus will be subjected, the bridge is arranged to be balanced andresistances R and S are conveniently arranged to have values of theorder of 20,000 ohms each. There is then zero feedback from anode togrid via the feedback path. At the highest expected, ambienttemperature, the static resistance R may maintain its 20,000 ohmvalueywhile resistance S falls to a value of the order of 15,000 ohmswith the result that the feedback is made negative.

and its associated by- 4 that there will be a corresponding slightoverall positive loss/temperature co-eficient for the pilot receivingequipment.

It will be understood that the invention is not limited to theparticular method shown of producing the required amplifiergain/temperature characteristic.

' What we claim as new and desire to secure by Letters Patent is:

1. In a telecommunication system, a compensating arrangement comprisingin combination a communication channel, a pilot frequency oscillatorconnected to one end of said channel, a filter circuit having a positiveloss/temperature coefiicient, said filter circuit being sharply tuned tothe-frequency of said pilot frequency'oscillator and connectedto theother end of said channel,

an amplifier for amplifying said pilot frequency after its passagethrough said filter, temperatureresponsive means in said amplifier,means controlled by said temperature-responsive means for automaticallyaltering the gain of said amplifier so as to compensate for theincreased loss in said filter with increase of temperature, and anindicator connected to the output of said amplifier for indicating thelevel of said pilot frequency.

2. In a telecommunication system, a compensating arrangement comprisingin combination a communication channel, a pilot frequency oscillatorconnected. to one end of said channel, a, filter circuit having apositive loss/temperature coefi'lcient, said filter circut being sharplytuned to the frequency of said pilot frequency oscillator and connectedto the other end of said channel,

an amplifier including a thermionic valve with negative feedback foramplifying said pilot frequency after its passage through said filter,temperature-responsive means in said amplifier, and means controlled bysaid temperature responsive means for automatically altering the amountof feedback to said valve so that with increase of temperature theincrease of gain of said amplifier compensates for the increased loss insaid filter.

3. A compensating arrangement in accordance with claim 2 in which thesense and amount of the feedback is controlled by a bridge arrangementemploying two resistances composed respectively of materials havingappreciably different resistance/temperature coefficients.

4. A compensating arrangement in accordance with claim 2 in which thesense and amount of the feedback is controlled by abridge arrangementemploying two resistances, one resistance

